1. Field of the Invention
The present invention relates to an evaporated fuel treatment device of an engine.
2. Description of the Related Art
Known in the art is an internal combustion engine provided with a canister for temporarily storing evaporated fuel and a purge control valve for controlling the amount of purge of the fuel vapor to be purged from the canister to the inside of an intake passage and using the purge control valve to control the amount of purge of the fuel vapor so that the purge rate of the fuel vapor becomes a predetermined target purge rate (see Japanese Unexamined Patent Publication (Kokai) No. 7-305646). In this internal combustion engine, the amount of fuel injection is corrected by a feedback correction coefficient and a purge air fuel ratio correction coefficient so as to suitably maintain the air-fuel ratio at the target air-fuel ratio.
That is, in a usual internal combustion engine, the basic fuel injection time required for making the air-fuel ratio the stoichiometric air-fuel ratio is found in advance by experiments and stored and the basic fuel injection time is corrected by a feedback control coefficient so that the air-fuel ratio becomes the stoichiometric air-fuel ratio based on an output signal of an air-fuel ratio sensor provided in the engine exhaust passage. In this case, the feedback control coefficient is normally set to repeatedly fluctuate about a reference value, for example, 1.0.
On the other hand, when the purge action of fuel vapor is started, the basic fuel injection time is corrected by the purge air-fuel ratio correction coefficient showing the vapor concentration in the intake air. The feedback control coefficient repeatedly fluctuates about 1.0 even during the purge action. That is, when the purge action is started, the air-fuel ratio becomes rich, so the feedback control coefficient falls to where the air-fuel ratio becomes the stoichiometric air-fuel ratio. The amount of drop of the feedback control coefficient at this time shows the vapor concentration In the intake air. Therefore, to find the vapor concentration there, the purge air-fuel ratio correction coefficient is gradually updated until the feedback control coefficient returns to 1.0. In this case, the final purge air-fuel ratio correction coefficient comes to show the vapor concentration and the fuel injection time is shortened by exactly the amount of the final purge A/F correction coefficient. When the final purge A/? correction coefficient is calculated, the feedback control coefficient once again fluctuates about 1.0.
Note that in actuality, the narrow range about 1.0 is set for the feedback control coefficient for calculating the vapor concentration. Only when the feedback control coefficient exceeds this set range is the purge A/F correction coefficient reduced or increased so that the feedback control coefficient returns to the set range. By correcting the amount of fuel injection by the feedback control coefficient and the vapor air-fuel ratio correction coefficient in this way, the air-fuel ratio is maintained at the stoichiometric air-fuel ratio even when a purge action is being performed.
On the other hand, if the feedback control coefficient deviates widely from 1.0 due to some reason or another, the air-fuel ratio will become excessively lean or excessively rich. Therefore, it is necessary to prevent the feedback control coefficient from deviating widely from 1.0 in this way. Therefore, usually, an allowable limit of fluctuation is set for the feedback control coefficient and the feedback control coefficient is made to only be able fluctuate within this allowable limit of fluctuation.
Since, however, the fuel vapor is sucked into the intake passage by the difference in the pressure in the canister and the pressure inside the intake passage, if the degree of opening of the purge control valve is made constant, the larger the vacuum in the intake passage, the greater the purge rate will become. Therefore, in this case, to maintain the purge rate constant, the greater the vacuum in the intake passage, the smaller the degree of opening of the purge control valve must be made. Accordingly, in the past, to maintain the purge rate at the target purge rate, the degree of opening of the purge control valve has been controlled in is accordance with the operating state of the engine.
The fuel vapor absorbed in the activated charcoal in the canister, however, is sucked into the intake passage by the difference between the pressure in the canister and the pressure in the intake passage, so if the degree of opening of the purge control valve were controlled in accordance with the operating state of the engine in the above way when the fuel vapor absorbed in the activated charcoal of the canister was being purged, it would be possible to maintain the purge rate at the target purge rate and therefore prevent the air-fuel ratio from fluctuating.
A little after the engine starts operating, however, the rise of the temperature of the fuel in the fuel tank and vibration cause a large amount of evaporated fuel to be generated in the fuel tank. The evaporated fuel generated in the fuel tank is made to flow through a purge passage directly into the intake passage. In this case, however, since the fuel vapor is pushed out from the fuel tank into the intake passage, the amount of the fuel vapor supplied from the fuel tank to the intake passage is not dependent on the magnitude of the vacuum generated in the intake passage, but is dependent on the amount of the evaporated fuel generated in the fuel tank. Therefore, if the amount of the fuel vapor which is directly supplied from the fuel tank to the inside of the intake passage increases, even if the purge control valve is used to control the purge rate to a target purge rate, the concentration of fuel vapor in the intake air will greatly change in accordance with a change in the amount of purge.
That is, when the purge rate is controlled to become the target purge rate, if the amount of intake air falls, the amount of purge will be reduced, while if the amount of intake air increases, the amount of purge will be increased. In this case, if the amount of purge is increased, the amount of purge of the fuel vapor from the canister will be increased in exact proportion and the amount of purge of the fuel vapor from the fuel tank will be maintained constant. Therefore, if the amount of purge is increased, the concentration of fuel vapor in the intake air will become lower. If the amount of fuel vapor directly supplied from the fuel tank to the inside of the intake passage is increased in this way, when the amount of purge increases, the concentration of fuel vapor in the intake air will become lower, while when the amount of purge decreases, the concentration of fuel vapor in the intake air will become higher.
Note that even when the concentration of fuel vapor fluctuates widely in accordance with the changes in the amount of purge in this way, if the feedback control coefficient is changed to track these changes of the concentration of fuel vapor, the air-fuel ratio will not fluctuate that much. That is, for example, if the amount of intake air falls and the amount of purge falls like at the time of a deceleration operation, the concentration of fuel vapor will widely increase and therefore the feedback control coefficient will become smaller to maintain the air-fuel ratio at the stoichiometric air-fuel ratio. At this time, if the feedback control coefficient can be kept smaller until the air-fuel ratio reaches the stoichiometric air-fuel ratio, there will be almost no fluctuation in the air-fuel ratio.
In the past, however, as explained above, when the feedback control coefficient exceeded a set range, the feedback control coefficient was returned to a set range about 1.0. That is, for example, even when the amount of purge was small and therefore the concentration of fuel vapor was high, the feedback control coefficient was returned to within the set range around 1.0. Therefore, when an acceleration operation was performed from this state and the amount of purge increased, the concentration of fuel vapor widely fell, so the feedback control coefficient was increased from near 1.0 to maintain the air-fuel ratio at the stoichiometric air-fuel ratio. At this time, however, since the amount of fluctuation of the concentration of fuel vapor is large, the feedback control coefficient reaches the allowable limit of fluctuation and can no longer increase any further. That is, the feedback control coefficient is made not to be able to increase up to where the air-fuel ratio becomes the stoichiometric air-fuel ratio. As a result, the air-fuel ratio becomes considerably lean and therefore a good acceleration operation cannot be obtained. Not only this, but there is also the problem that the exhaust emission deteriorates.
A similar situation occurs even when the amount of purge is reduced as at the time of deceleration operation. That is, even when the amount of purge is large and the concentration of the fuel vapor is low, the feedback control coefficient is returned to within the set range near 1.0. Therefore, if a deceleration operation is performed from this state and the amount of purge is reduced, the concentration of the fuel vapor is greatly increased, so the feedback control coefficient ends up falling from near 1.0 to maintain the air-fuel ratio at the stoichiometric air-fuel ratio. At this time, however, the amount of fluctuation of the concentration of the fuel vapor is large, so the feedback control coefficient reaches the allowable limit of fluctuation and then can no longer fall further. That is, the feedback control coefficient cannot become small enough so that the air-fuel ratio is made the stoichiometric air-fuel ratio. As a result, the air-fuel ratio becomes considerably rich and therefore the problem arises that the exhaust emission deteriorates.
On the other hand, the air-fuel ratio does not only fluctuate when a large amount of evaporated fuel is produced in the fuel tank as explained above. The air-fuel ratio also fluctuates widely due to changes in the state of driving of the vehicle.
That is, in an internal combustion engine designed so as to control the purge rate of the fuel vapor to become a predetermined target purge rate, for example, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 6-146965, when the air-fuel ratio deviates from the target air-fuel ratio, the calculated value of the concentration of the fuel vapor is updated by exactly a predetermined set amount without regard to the purge rate and the amount of fuel injection is corrected bared on the updated concentration of the fuel vapor so that the air-fuel ratio becomes the target air-fuel ratio.
When the air-fuel ratio deviates from the target air-fuel ratio in this way, however, if the concentration of the fuel vapor is updated by exactly a predetermined set amount without regard to the purge rate in this way, in particular when the purge rate becomes larger from a small state, the problem arises that the air-fuel ratio ends up deviating from the target air-fuel ratio.
That is, the air-fuel ratio does not fluctuate due to just the effect of the purge action. It also fluctuates due to the change in the state of driving of the vehicle. Accordingly, if the deviation of the air-fuel ratio is considered to be due entirely to the effect of the purge action and the amount of deviation of the air-fuel ratio is reflected back into the update value of the concentration of fuel vapor, the concentration of fuel vapor which is calculated will end up deviating from the actual concentration of fuel vapor. If the concentration of fuel vapor which is calculated deviates from the actual concentration of the vapor in this way, there will be no problem when the purge rate does not change or the purge rate becomes smaller, but there will be a problem when the purge rate becomes larger than a small value.
That is, for example, assume that the air-fuel ratio deviates by 2 percent from the target air-fuel ratio due to the change in the state of driving of the vehicle, not the effect of the purge action, and that the purge rate is a small value, for example, 0.5 percent. At this time, if the deviation of the air-fuel ratio is considered to be all due to the effect of the purge action and the amount of deviation of the air-fuel ratio is reflected back into the update value of the concentration of the fuel vapor, the vapor concentration which is calculated will end up deviating by 4 percent per unit purge rate (2%/0.5%) with respect to the actual concentration of the vapor. In this case, if the purge rate is maintained at 0.5 percent, the calculated vapor concentration will continue to deviate by 2 percent from the actual vapor concentration.
If the purge rate increases, however, for example, if the purge rate rises from 0.5 percent to 5 percent, the amount of deviation of the calculated vapor concentration becomes 20 percent (4% amount of deviation per unit purges rate x purge rate 5%). If the amount of deviation of the vapor concentration calculated becomes 20 percent, the amount of fuel supplied corrected based on the calculated vapor concentration will deviate considerably from the amount of fuel supply necessary for maintaining the target air-fuel ratio and therefore the problem will arise of the air-fuel ratio deviating considerably from the target air-fuel ratio.
On the other hand, when the air-fuel ratio deviates by 2 percent from the target air-fuel ratio due to the effect of the state of driving of the vehicle and the purge rate is a large value, for example, 5 percent, the vapor concentration which is calculated will be only 0.4 percent per unit purge rate (2%/5%). Therefore, at this time, there is little error in the vapor concentration and no particular problem. Further, when the purge rate falls from this state, the amount of deviation of the concentration of vapor will gradually become smaller, so there will be no particular problem in this case as well. Accordingly, a problems occurs only when updating the concentration of the fuel vapor when the purge rate is low.